Low heat expansion alloy

ABSTRACT

It is an objective of the present invention to provide low thermal expansion alloys having excellent high temperature mechanical properties, the oxidation resistance, and the electrical conductivity by modifying the basic Cr--W--Fe alloy in order to meet various industrial demands including prolonging the life of plant machinery being operated at high temperature and a scaling-up solid oxide fuel cell, so that the coefficient of thermal expansion can be approximated to those of stabilized zirconia. In order to achieve these objectives, Co is added to Cr--W--Fe system alloy possessing an excellent thermal matching characteristic to stabilized zirconia in order to enhance the high temperature mechanical properties without changing original properties of thermal matching, the oxidation resistance and the electrical conductivity. Cr and Al are added in order to further improve the oxidation resistance and to approximate the coefficient of surface thermal expansion to that of stabilized zirconia. Furthermore, by adding at least one element properly selected from the element group comprising of Ti, Zr, and Hf, the electrical resistance can be reduced; if Hf element is co-added with B element, the hot working characteristics can be especially improved. Any combination of Co, Al, and (Ti,Zr,Hf) can be appropriately chosen and added to the basic Cr--W--Fe system alloys. B element can be added in order to prevent the grain boundary segregation of W element. In order to improve a further resistance against the high temperature oxidation, one or more than two elements properly selected from the element group comprising of Y, Hf, Ce, La, Nd, and Dy can be added to the basic Cr--W--Fe system alloy.

This application is a 371 of PCT/JP98/01121 filed Mar. 17, 1998.

DESCRIPTION

1. Technical Field

The present invention relates to an improvement of the low thermalexpansion alloys composed of the basic Cr--W--Fe system alloy by whichthe coefficients of thermal expansion of said alloy are approximated tothe coefficient of thermal expansion of stabilized zirconia. The presentinvention is furthermore aimed on several objectives. These objectivesmay include (1) an improvement of oxidation resistance by forming asurface oxide layer consisting of Cr₂ O₃ containing Al₂ O₃ on theextremely outer surface and an inner layer with dispersed Al₂ O₃, (2) anenhancing mechanical properties by adding Co element, (3) a reducingspecific resistance by adding at least one element properly selectedfrom the element group consisted of Ti, Zr, and Hf, (4) an adding Belement in order to prevent the grain boundary segregation of W element,and (5) an adding M element in order to improve the oxidationresistance; where said M element is appropriately selected one or morethan two elements from the element group consisting of Y, Hf, Ce, La,Nd, and Dy. Then, by accomplishing the aforementioned objectives, thepresent invention directly relates to provide an alloy with lowcoefficient of thermal expansion, which is furthermore characterized by(a) a formation of an intermediate layer which serves as a thermalbarrier coating and is formed between zirconia system ceramic andsubstrate material. The low thermal expansion alloy is, moreover,characterized by (b) a coefficient of thermal expansion which isapproximated to the coefficient of thermal expansion of stabilizedzirconia--which is a promising third generation ceramic materialapplicable as solid oxide fuel cell, and (c) some other excellentphysical and mechanical properties including high-temperature strength,oxidation resistance and low specific resistance.

2. Background Art

Recently, there is a significant trend of more severe operationalconditions of higher temperature and higher pressure in a lot of plantmachinery being operated at high temperature regimes including agenerator gas turbine, reflecting to saving energy and globalenvironmental issues. As a result, metallic materials which have beenused under the critical conditions are subjected to a severe extent ofdamage and deterioration processes.

Hence an anti-corrosion coating is normally applied to surfaces ofstatic and dynamic blades made of high strength superalloys in gasturbines for both airplanes and on-ground applications. However, theanti-corrosion coating is not a perfect solution to solve the currentdamage and deterioration problem due to a high temperature corrosion.

By the thermal barrier coating (TBC) as an example, the ceramic materialhaving a lower thermal conductivity is coated onto the metallic materialwith temperature gradient in order to prevent unwanted temperature riseon the metal surface layer. The thermal barrier coating has been appliedto the fuel chamber in the gas turbine system for more than 10 years,and recently is applied to cooling blades, too. The test conducted on areal blade indicated that the thermal barrier effect was observed with atemperature ranging from 50° C. to 100° C.

The thermal barrier coating is consisted of mainly two parts; the firstlayer is a ceramic sprayed layer made of mainly ZrO₂ which is asolid-solution with a stabilizer such as MgO, Y₂ O₃, CaO or the like,and has a heat conductivity of 0.005˜0.006 (cal/cm.s.°C.), being muchless than those values for Al₂ O₃ (0.04˜0.08) or TiO₂ (0.01˜0.02). Thesecond layer is an intermediate sprayed layer composed of Ni--Al systemalloy, Ni--Cr system alloy or M--Cr--Al--Y system alloy (whereas Mrepresents Fe, Ni, Co element or the like). The second intermediatelayer is formed for a purpose of reducing the thermal expansiondifference between said ceramic sprayed layer and alloy substrate, andof improving corrosion resistance. This intermediate layer can befabricated as a multi-layer structure of the mixed layer of said metaland ceramic materials. As an alternative approach, the intermediatelayer can be formed as a continuously gradient layer in terms ofcompositions, so that it can also perform as a functionally gradientstructure.

On the other hand, the fuel cell which receives an attention as a newtype of generating system has several types including phosphoric acidfuel cell (PAFC) using a phosphoric acid solution as an electrolyte,molten carbonate fuel cell (MCFC) using a potassium carbonate, lithiumcarbonate or the like as an electrolyte, and solid oxide fuel cell(SOFC) using a zirconia system ceramic as an electrolyte. Any one ofthese types possesses excellent uniqueness of generating technology by adirect converting from chemical energy of fuel material to electricalenergy through electrochemical reactions.

Current situation based on an energy policy and global environmentalconcern requires a rapid development of a dispersed type electric sourcewhich can be built at any location being close to the demanding placeand a fuel cell as a co-generating electric source. Moreover, a highergenerated power of the dispersed type electric source expects much forthe fuel cell.

By solid oxide fuel cell, a single cell is formed by sandwiching bothsides of the electrolyte plates made of yttria stabilized zirconia (YSZ)with a fuel electrode (as an anode) and an air electrode (as a cathode).Solid oxide fuel cell is, furthermore, constructed as a form of amulti-layer structure of a plurality of said single cells through aseparator in order to obtain a practical level of electrical power. H₂and CO are supplied to a passage space between said separator and thefuel electrode (anode), and the air is introduced into the passage spaceformed between the separator and the air electrode (cathode), so that agenerating system can be established by using reverse reaction ofelectrolysis of the water.

The development target of the high temperature gas turbine proposed inthe so-called Moon-Light Plan aiming toward an Energy-Saving Issuepromoted by the Japanese Ministry of International Trade and Industry(MITI) is to reach the final inlet gas temperature of 1,773K and toapproach the 55% of the total generating power efficiency by aco-generating system which is combined with a steam turbine driven bythe heat exhausted from the turbine.

The current level of the power generating efficiency of thermal powerplants by operating steam turbines is about 40%. If this level of powergenerating efficiency is increased by 10%, it is estimated that the fuelof approximately 2.2 billion dollars (based on 140 Japanese Yen=1 USdollars) can be saved in Japan per a single year.

By a structure of the aforementioned thermal barrier coating (TBC), apresence of the sprayed layer of stabilized zirconia is the mostimportant component. As described previously, Ni-based superalloys areutilized in order to approach the target of higher temperature andhigher pressure operation conditions. If the Ni-based superalloy is usedwithout any surface protective coating in the gas turbine atmosphere, alife more than one year can not be expected. Therefore, it isindispensable to have a protective coating on surface layers of the gasturbine blade materials.

However, since there is a large discrepancy in the coefficients ofthermal expansion between stabilized zirconia (10˜12×10⁻⁶ /K) andNi-based superalloys (18˜20×10⁻⁶ /K), the sprayed layer of stabilizedzirconia tends to have cracks. This is one of the major technicaldrawbacks associated with the conventional type of coating.

On the contrary to the above, an intermediate layer can be sprayed inorder to reduce the significant difference in the coefficients ofthermal expansion, and said intermediate layer is consisted of Ni--Alsystem alloy, Ni--Cr system alloy, or M--Cr--Al--Y system alloy (whereasM represents Fe, Ni, Co element or the like), expecting an improvedcorrosion resistance, too. However, the coefficients of the thermalexpansion of these intermediate layers are about 16˜18×10⁻⁶ /K, whichare still too high. Hence the problem of the thermal mismatching has notbe solved yet.

Now moving to our attention to solid oxide fuel cell, a presence of theseparator is the most important component here. The fuel cell isnormally constructed with a multiple-layer planar structure in order toenhance the effective surface area of the electrode and to reduce theinternal resistance.

Since several properties are required to the fuel cell separator, (La,alkaline earth metals)CrO₃ is normally selected as a separator material.The required properties are (1) a thermal expansion coefficient whichshould be ideally close to those values of the coefficient of thermalexpansion of the air electrode, the fuel electrode or the solidelectrolyte, (2) a high corrosion resistance, and (3) a high electricalconductivity.

A practical role of the separator is to separate the individual cellswhen building a multiple-layer structure, to shield the air from H₂fuel, and to support electrolyte plates.

In order to support the electrolyte plates, the surface area of theelectrolyte plate should be fabricated with a bigger size than thesurface area of the fuel electrode and the air electrode, so that themultiple-layer formation with the separator can be easily achieved andthe electrolyte plates can be easily supported. However, the separatoris made of the ceramic material as mentioned previously, it has severaltechnical problems; including (1) a relatively weak mechanical strength,(2) a poor formability, and (3) less ability to scale-up the structure.

Because the separator is needed to connect between the air electrodewhich is exposed to an oxidation atmosphere and the fuel electrode whichis exposed to a reduction atmosphere at high temperatures, the separatormaterial should possess high mechanical strengths in both oxidation andreduction atmospheres and a good electrical conductivity.

It has been proposed that LaCr₀.9 Mg₀.1 O₃, CoCr₂ O₄, or Ni--Al alloycan be used as a separator material. However, it was found that aconnection of these separators to the fuel electrode or the solidelectrolyte is not easily achieved.

A powder refining method has not been established for preparing uniformraw powder which is used for fabricating (La, alkaline earthmetals)CrO₃. Although refractory alloys including stainless steels orInconel exhibit superior mechanical strengths to the aforementionedceramic materials, solid electrolyte is subjected to a tensile stress atthe cell operating temperature (which is about 1000° C. (=1,273K) due toa large coefficient of thermal expansion of the aforementioned metals.The values of electrical resistance of oxide films formed on theserefractory alloys are relatively high; which is another technicaldisadvantage.

With metallic separator, there are problems, including a mismatching incoefficient of thermal expansion, and a growth of oxides formed on therefractory steels. In order to solve the thermal mismatching problem,there are several ideas proposed; including (1) employing a foamedstructure of LnMnOx for a connector, or (2) approaching the coefficientof thermal expansion by manipulating chemical compositions of metals.Furthermore, for a countermeasure of the oxide film growth problem, aspraying of LaCrO₃ has been tried. However, none of these ideas appearsto be satisfactory.

There has been great efforts in research and development inmanufacturing the stabilized zirconia to which MgO, Y₂ O₃, CaO or thelike is mixed as a solid-solution type stabilizer. These activities arepromoted because of unique properties of stabilized zirconia such ashigh mechanical strength and toughness, high melting point and heatresistance, and electrical characteristics. Stabilized zirconia has beenemployed in various sectors in industries such as a steel industry, achemical industry, a battery manufacturing, spray materials, turbine,internal combustion engine, sensors, and many others' applications.

Except the case when zirconia used as a ceramic itself, in manyapplications the ceramic is used by connecting to the adjacent metallicmaterials. However, there has not been developed any satisfactoryceramics which have a close value of the coefficient of thermalexpansion to those of metals and applicable in various industries.

The present inventors have previously proposed Cr--W--M--Fe system alloyand Cr--W--M--B--Fe system alloy which are designed and fabricated byadding an M element (where M represents one or more than two elementsproperly selected from the element group comprising of Y, Hf, Ce, La,Nd, and Dy) to the basic Cr--W--Fe system alloy or adding B element tothe basic Cr--W--M--Fe system alloy (Japan Patent Application Laid-OpenNo. Tokkai Hei 8-277441; 1996).

It was found that these alloys exhibit a close value of coefficient ofthermal expansion to those of stabilized zirconia and an excellentoxidation resistance at elevated temperatures.

Namely, the coefficients of thermal expansion of the aforementionedalloys are in a range of 12˜13×10⁻⁶ /K, which are very close to thecoefficient of thermal expansion of stabilized zirconia (10˜12×10⁻⁶ /K).Moreover, it was also found that the oxidation resistance of thesealloys shows a superior property to those of conventional stainlesssteels. However, these newly proposed alloys are not perfectlysatisfactory due to the ever-increasing demands on higher temperatureand higher pressure operation conditions, requiring more oxidationresistance and a closer value of coefficient of thermal expansion toexhibit excellent thermal matching to stabilized zirconia.

Furthermore, since the aforementioned alloys have larger values ofspecific resistance than those of conductive ceramic, it is difficult toobtain an excellent electrical conductivity. Moreover, the mechanicalstrength of these alloys is not strong enough as to a structuralmaterial; In particular, the high temperature strength (about 1,273K) ofthese alloys is not satisfactory.

THE OBJECTIVES OF THE INVENTION

As a consequence, in order to overcome the problems associated with theconventional type of materials, it is, therefore, an objective of thepresent invention to provide an oxidation resisting low thermalexpansion alloy which has a close value of coefficient of thermalexpansion to that of stabilized zirconia and possesses excellentoxidation resistance and electrical properties in order to meetrequirements for prolonging lives of various types of high temperatureplant machinery including gas turbines and for scaling-up solid oxidefuel cell.

It is another objective of the present invention to provide a highstrength low thermal expansion alloy which has a close value ofcoefficient of thermal expansion to that of stabilized zirconia andexcellent high temperature strength.

It is a further objective of the present invention to provide a lowthermal expansion alloy with a low specific resistance.

It is yet another objective of the present invention to provide a lowthermal expansion alloy which is further characterized by combinedproperties of the aforementioned excellent oxidation resistance, hightemperature strength, and low specific resistance.

DISCLOSURE OF INVENTION

Based on the contents described in our previous proposed idea (JapanPatent Application Laid-Open No. Tokkai Hei 8-277441; 1996), the presentinventors have conducted various investigations; aiming to developmetallic materials which possess excellent thermal matchingcharacteristics, an oxidation resistance, and an electrical conductivityas well. Our continuous and diligent efforts for the research haveresulted in the fact that when Al element is added to the Cr--W--Fesystem alloy, Al element is selectively oxidized due to a higheraffinity to oxygen than Cr element does, so that an oxide layer beingconsisted of Cr₂ O₃ with Al₂ O₃ formed on surface layer of the metallicmaterial and Al₂ O₃ is dispersed inside the metal substrate due tointernal oxidation, by controlling properly adding amounts of Cr and Alelements.

The oxide layer of Cr₂ O₃ containing Al₂ O₃ formed on the surface layerwas found to exhibit better protective properties than Cr₂ O₃ aloneformed on metal surface as proposed in the previous application. Also itwas found that the evidence of containing Al₂ O₃ contributes in reducingoxidation rate. As a result, the oxidation resistance is enhanced. Onthe contrary, if the surface layer of the metal substrate is coveredwith Al₂ O₃ only, the electrical resistance will increase, and thecoefficient of thermal expansion will increase with increasing Alcontents. Therefore, metallic material being added with only Al elementcan not be used as an intermediate layer between the thermal barriercoating and the substrate, nor a separator for solid oxide fuel cellwith an electrolyte of stabilized zirconia.

After conducting research on interrelationships between Al addition andoxidation resistance, electrical conductivity, or thermal expansivity,then, the present inventors have found that adding Al element with acertain amount to the basic Cr--W--Fe system alloy forms Cr₂ O₃containing Al₂ O₃ on the surface layer of the metallic material.Furthermore, it was found that the thus formed duplex oxide layerexhibits superior protectiveness and oxidation resistance to Cr₂ O₃alone formed on Cr--W--Fe system alloy. Moreover, since the oxide layerthickness can be thinner, so that the excellent electrical conductivitycan be also obtained. The presence of internally dispersed Al₂ O₃ makesthe material's coefficient of thermal expansion further close to that ofstabilized zirconia. Furthermore, the present inventors have completedthe first invention that, in addition to the aforementioned improvement,adding B element to Cr--W--Al--Fe system alloy prevents the grainboundary segregation of W element and adding at least one elementproperly selected from the element group comprising of Y, Hf, Ce, La,Nd, and Dy to any one of the above described alloys enhances furthermorethe oxidation resistance.

Based on our previous application (Japan Patent Application Laid-OpenNo. Tokkai Hei 8-277441; 1996), the present inventors have investigatedto further develop new alloy systems which can show much better thermalmatching characteristic, oxidation resistance and electricalconductivity, as well. Especially better mechanical properties at hightemperature regimes were our primary goal. It was found that when Coelement is added to Cr--W--Fe system alloy, the high temperaturemechanical properties were improved with maintaining still excellentthermal matching characteristic, the oxidation resistance, and theelectrical conductivity.

When a small amount of Al is added to Cr--W--Co--Fe system alloy, Al isselectively oxidized because Al element has a higher affinity to oxygenthan Cr element does. Hence, by controlling properly adding amounts ofCr and Al elements, Cr₂ O₃ containing Al₂ O₃ will be formed on thesurface layer of the metallic substrate and Al₂ O₃ is internally formedand dispersed inside the metallic substrate, so that a furtherresistance against the high temperature oxidation is improved. Since theoxide layer thickness is also thinner, an excellent electricalconductivity can be achieved. It was also found that the presence of theinternally dispersed Al₂ O₃ oxide makes the metallic material'scoefficient of thermal expansion further closer to that of stabilizedzirconia.

Furthermore, the present inventors have observed that adding a smallamount of B element to Cr--W--Co--Fe system alloy and Cr--W--Co--Al--Fesystem alloy prevents the grain boundary segregation of W element.Addition to this, it was found that adding at least one element properlyselected from the element group comprising of Y, Hf, Ce, La, Nd, and Dyto any one of the aforementioned alloy systems improves furtherresistance against the high temperature oxidation, resulting in the newsecond invention.

The present inventors have also conducted tests to develop new Cr--W--Fealloy systems which possess a low specific resistance. It was found thatadding at least one element properly selected from the element groupcomprising of Ti, Zr, and Hf reduces the specific resistance withoutdeterioration of the original thermal matching characteristic and theoxidation resistance.

When a small amount of Al element is added to the above mentionedCr--W--(Ti,Zr,Hf)--Fe system alloy, surface oxide layer of Cr₂ O₃containing Al₂ O₃ is formed by appropriate controlling adding amounts ofboth Cr and Al elements, owing to the fact that Al--which has a higheraffinity to oxygen than Cr does--is selectively oxidized. Thus formedsurface duplex oxide layer exhibits a superior protectiveness to thesurface oxide of Cr₂ O₃ alone formed on Cr--W--Fe alloy and the oxidelayer thickness is thinner, so that the excellent electricalconductivity can be achieved. By a presence of the internally dispersedAl₂ O₃, the coefficient of thermal expansion at the surface layer areawill be further closer to that of stabilized zirconia.

Moreover, when Co element is added to Cr--W--(Ti,Zr,Hf)--Fe system alloyor Cr--W--(Ti,Zr,Hf)--Al--Fe system alloy, it was found that the hightemperature mechanical properties will be improved with maintainingoriginal excellent thermal matching characteristic, the oxidationresistance, and the electrical conductivity as well.

Furthermore, when the present inventors added a small amount of Belement to Cr--W--(Ti,Zr,Hf)--Fe system alloy, Cr--W--(Ti,Zr,Hf)--Al--Fesystem alloy, Cr--W--(Ti,Zr,Hf)--Co--Fe system alloy, andCr--W--(Ti,Zr,Hf)--Al--Co--Fe system alloy, it was found that the grainboundary segregation of W element is prevented. By adding at leat oneelement properly selected from the element group comprising of Y, Hf,Ce, La, Nd, and Dy to any one of the above mentioned alloys systems,further improvement of the oxidation resistance was recognized. If Hfelement is added, the hot working characteristics is particularlyfurther improved under the co-existence of B element, resulting in thethird invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The oxidation resisting low thermal expansion alloy, according to thepresent invention, is characterized by the fact that said alloy containsCr 15˜40 wt % (weight %), W 5˜15 wt %, Al 0.1˜1 wt %, and is balanced byFe with unavoidable impurities.

The high strength low thermal expansion alloy, according to the presentinvention, is characterized by the fact that said alloy contains Cr15˜40 wt %, W 5˜15 wt %, Co 1˜10 wt %, and is balanced by Fe withunavoidable impurities.

Moreover, the low specific resistance low thermal expansion alloy,according to the present invention, is characterized by the evidencethat said alloy contains Cr 15˜40 wt %, W 5˜15 wt %, at least oneelement properly selected from the element group comprising of Ti, Zr,and Hf with 0.1˜5 wt % (if Hf is solely selected, it should be 1˜5 wt%), and is balanced by Fe with unavoidable impurities.

To any one of aforementioned Cr--W--Fe alloy systems, the followingalloying element(s) can be selected to make further characteristicalloy(s); including Al with 0.1˜1 wt % in order to improve the oxidationresistance, Co with 1˜10 wt % in order to enhance the mechanicalstrengths, at least one element properly selected from the element groupcomprising of Ti, Zr, and Hf with 0.1˜5 wt % in order to reduce thespecific resistance, B with 0.001˜0.01 wt % in order to prevent thegrain boundary segregation of W of these alloys, and M (M can be one ormore than two elements properly selected from the element groupcomprising of Y, Hf, Ce, La, Nd, and Dy) with 0.01˜1 wt % in order tofurther improve the oxidation resistance.

Namely, there are various systems of low thermal expansion alloys, aslisted below. However, if the Hf element is chosen for reducing thespecific resistance, it is not necessary to select Hf element from the Mgroup in order to improve the oxidation resistance.

Cr--W--Al--Fe system alloy

Cr--W--Al--B--Fe system alloy

Cr--W--Al--M--Fe system alloy

Cr--W--Al--B-M--Fe system alloy

Cr--W--Co--Fe system alloy

Cr--W--Co--B--Fe system alloy

Cr--W--Co--M--Fe system alloy

Cr--W--Co--B-M--Fe system alloy

Cr--W--Co--Al--Fe system alloy

Cr--W--Co--Al--B--Fe system alloy

Cr--W--Co--Al--M--Fe system alloy

Cr--W--Co--Al--B--M--Fe system alloy

Cr--W--(Ti,Zr,Hf)--Fe system alloy

Cr--W--(Ti,Zr,Hf)--B--Fe system alloy

Cr--W--(Ti,Zr,Hf)--M--Fe system alloy

Cr--W--(Ti,Zr,Hf)--M--Fe system alloy

Cr--W--(Ti,Zr,Hf)--B--Fe system alloy

Cr--W--(Ti,Zr,Hf)--Al--B--Fe system alloy

Cr--W--(Ti,Zr,Hf)--Al--M--Fe system alloy

Cr--W--(Ti,Zr,Hf)--Al--B--Fe system alloy

Cr--W--(Ti,Zr,Hf)--Co--B--Fe system alloy

Cr--W--(Ti,Zr,Hf)--Co--M--Fe system alloy

Cr--W--(Ti,Zr,Hf)--Co--M--Fe system alloy

Cr--W--(Ti,Zr,Hf)--Co--B--Fe system alloy

Cr--W--(Ti,Zr,Hf)--Al--Co--B--Fe system alloy

Cr--W--(Ti,Zr,Hf)--Al--Co--B--M--Fe system alloy

Cr--W--(Ti,Zr,Hf)--Al--Co--M--Fe system alloy

Cr--W--(Ti,Zr,Hf)--Al--Co--B--M--Fe system alloy

In the followings, the containing limitations for each alloying elementwill be explained in details.

Cr is a basic alloying element in order to obtain the certain level ofthe heat resistance. Hence, it is necessary to contain at least 15 wt %.However, if it exceeds 40 wt %, the alloying effect will be saturated,rather the coefficient of thermal expansion will increase and theformability will be deteriorated. Consequently, it should be in a rangefrom 15 to 40 wt %; more preferably it should be 15˜25 wt %.

W is a basic alloying element in order to obtain a certain level of thecoefficient of thermal expansion, so that it is necessary to contain atleast 5 wt %. On the contrary, if it exceeds 15 wt %, the coefficient ofthermal expansion will increase, which is not favorable. As a result, itshould be in a range from 5 to 15 wt %; more preferably, it should be5˜10 wt %.

Co is a basic alloying element when high temperature strength isrequired. Therefore, it should be contained at least 1 wt %. On theother hand, if it exceeds 10 wt %, the hot working characteristics willbe deteriorated and the coefficient of thermal expansion will increase.Hence, it should be in a range of 1˜10 wt %; more preferably it shouldbe in a range from 5 to 10 wt %.

Al exhibits a significant alloying element to alloy systems of thepresent invention. Al can promote to form a surface oxide layer of Cr₂O₃ containing Al₂ O₃ and an internally dispersed Al₂ O₃ ; the formertype of external surface oxide layer will enhance the oxidationresistance and the latter type of internal oxides will make thematerial's coefficient of thermal expansion further closer to that ofstabilized zirconia. If Al content is less than 0.1 wt %, internallydispersed Al₂ O₃ oxides can not be formed. On the contrary, if itexceeds 1 wt %, the electrical conductivity will reduce and thecoefficient of thermal expansion will increase. Therefore, it should bein a range from 0.1 to 1 wt %.

At least one element properly selected from the element group comprisingof Ti, Zr, and Hf has an effect to reduce the electrical resistance. Ifit is less than 0.1 wt %, it can not show the efficiency; on the otherhand, if it exceeds more than 5 wt %, its effect will be saturated.Moreover, if Hf element is solely chosen, Hf with less than 1 wt % willnot exhibit its effect to reduce the electrical resistance. On the otherhand, if it exceeds more than 5 wt %, its effect will be saturated.Hence, it should be 0.1 (if Hf is selected, it should be 1 wt %)˜5 wt %;more preferably it should be 0.5˜2 wt %. For the Hf alone, it should bea range from 1 to 2 wt %.

B is very effective to prevent the grain boundary segregation of W, sothat it should contain at least 0.001 wt %. However, if it exceeds 0.01wt %, its effect will be saturated. Consequently, it should be0.001˜0.01 wt %. For the case when Hf contains more than 1 wt %, it willbe very effective for the hot working characteristics under aco-existence with the B element.

Any one or more than two elements properly selected from the elementgroup comprising of Y, Hf, Ce, La, Nd, and Dy will enhance the oxidationresistance. Hence it should contain at least 0.01 wt %. If it exceeds 1wt %, the hot working characteristics will be rapidly deteriorated. As aresult, it is recommended to add with a range from 0.01 to 1 wt %.

Although the low thermal expansion alloy according to the presentinvention is characterized by the fact that at least one elementproperly selected from the element group comprising of Ti, Zr, and Hf isadded to the Cr--W--Co--Fe system alloys in order to reduce theelectrical resistance, at least one element should be properly chosenand added out of the element group comprising of Y, Hf, Ce, La, Nd, andDy in order to add the improvement effect for the oxidation resistanceto the Cr--W--Co--(Ti,Zr)--Fe system alloy. Although the lowerelectrical resistance and higher oxidation resistance can be achieved byadding Hf to the Cr--W--Co--(Hf)--Fe system alloy, at least one elementproperly selected from the element group comprising of Y, Ce, La, Nd,and Dy can be added in order to further improve the resistance againstthe high temperature oxidation.

Fe is a basic element of any one of the aforementioned alloy systems andoccupies the balanced content.

The aforementioned alloy systems can be fabricated by an ordinarycasting technique. The casts can be further subjected to the hot or coldforming process or pulverized, depending on the desired final shape ofthe applications.

EMBODIMENTS Embodiment 1

In order to prove the predicted effect of the present invention, variousalloy systems according to the present invention were fabricated withchemical compositions as listed in Table 1. The coefficient of thermalexpansion from room temperature up to 1,273K and weight gain due to ahigh temperature oxidation, and increments in specific resistance weremeasured for all these alloys. All data obtained are listed and comparedto those obtained from the conventional types of alloys, as seen inTable 2. Data with * marks for the conventional materials indicate thatthese alloys are disclosed in the Japan Patent Application Laid-Open No.Tokkai Hei 8-277441 (1996).

The weight gain due to the high temperature oxidation was obtained fromthe weight differences between before and after divided by the totalexposed area. The high temperature oxidation was carried out in the airat 1,273K for 1,000 hours.

The increments in the specific resistance was obtained by increments inresistance multiplied by the total surface area of sample after beingexposed to the air at 1,273K for 490 hours. Since the oxide film on thespecimen has little thickness, the total surface area will be the mostimportant factor governing increments in the specific resistance. If themeasured resistance is R, R must be, therefore inversely proportional tothe surface area. Namely, R ∝1/S, where S is surface area. If theproportional constant to this equation is ρ, then we have R=ρ×(1/S),ρ=R·S. Hence, ρ has a unit of mΩ·cm².

From Tables 1 and 2, the low thermal expansion alloys according to thepresent invention exhibit close values of coefficient of thermalexpansion (10˜12×10⁻⁶ /K) of stabilized zirconia. Moreover, it was foundthat the alloy systems of the present invention demonstrated theirsuperior properties of electrical conductivity and oxidation resistanceto the conventional types of alloys.

                                      TABLE 1                                     __________________________________________________________________________    Cf. Inv.:This Invention, Com.:Comparison                                      Sam.   chemical composition(wt %, bal-Fe)                                     No.    Cr W  Y  Hf Ce La Nd Dy B  Al                                          __________________________________________________________________________    Inv.                                                                              1  15.2                                                                             13.3                                                                             -- -- -- -- -- -- -- 0.32                                            2  24.5                                                                             9.1                                                                              -- -- -- -- -- -- -- 0.60                                            3  30.7                                                                             7.7                                                                              -- -- -- -- -- -- 0.004                                                                            0.88                                            4  37.4                                                                             5.1                                                                              -- -- -- -- -- -- 0.008                                                                            0.13                                            5  16.8                                                                             7.0                                                                              -- -- -- -- -- 0.11                                                                             -- 0.43                                            6  39.9                                                                             6.1                                                                              -- 0.09                                                                             -- -- -- -- 0.007                                                                            0.27                                            7  17.7                                                                             14.8                                                                             -- -- -- -- 0.28                                                                             0.04                                                                             -- 0.56                                            8  23.8                                                                             5.2                                                                              -- -- -- 0.40                                                                             -- -- -- 0.22                                            9  15.4                                                                             8.3                                                                              -- -- -- -- 0.15                                                                             -- 0.002                                                                            0.34                                            10 17.5                                                                             9.5                                                                              -- -- 0.019                                                                            0.010                                                                            -- -- -- 0.11                                            11 33.8                                                                             7.7                                                                              0.91                                                                             -- -- -- -- -- 0.006                                                                            0.67                                            12 18.3                                                                             5.6                                                                              -- -- 0.007                                                                            0.003                                                                            -- -- -- 0.97                                            13 20.2                                                                             6.9                                                                              -- -- 0.161                                                                            0.073                                                                            -- -- -- 0.55                                        Com.                                                                              14 17.6                                                                             7.1                                                                              -- 0.08                                                                             -- -- -- -- 0.003                                                                            --                                              15 17.5                                                                             14.9                                                                             -- -- -- 0.32                                                                             -- -- 0.009                                                                            --                                              16 15.1                                                                             8.0                                                                              -- -- -- -- 0.16                                                                             -- -- --                                              17 24.2                                                                             5.1                                                                              -- -- -- -- 0.30                                                                             0.05                                                                             0.005                                                                            --                                              18 39.7                                                                             6.5                                                                              -- -- -- -- -- 0.12                                                                             -- --                                              19 10.3                                                                             6.9                                                                              -- -- -- -- -- -- -- 0.05                                            20 13.5                                                                             16.1                                                                             -- -- -- 0.17                                                                             -- -- -- --                                              21 17.8                                                                             9.8                                                                              0.95                                                                             -- -- -- -- -- -- --                                              22 18.0                                                                             5.2                                                                              -- -- 0.155                                                                            0.065                                                                            -- -- -- --                                              23 20.0                                                                             6.8                                                                              -- -- 0.008                                                                            0.002                                                                            -- -- -- --                                              24 34.5                                                                             7.4                                                                              -- -- 0.018                                                                            0.005                                                                            -- -- -- --                                              25 17.3                                                                             -- -- -- -- -- -- -- -- --                                              26  5.1                                                                             6.8                                                                              -- -- -- -- -- -- -- --                                              27 44.8                                                                             7.2                                                                              -- -- -- -- -- -- -- --                                              28 17.9                                                                             9.0                                                                              -- -- -- -- -- -- -- 1.44                                        __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________    Cf. Inv.:This Invention, Com.:Comparison                                                     weight                                                                        gain  the   the presence                                              coefficient                                                                           due to the                                                                          increments                                                                          of the                                                    of      high  in the                                                                              dispersed                                                 thermal temperature                                                                         specific                                                                            Al.sub.2 O.sub.3 oxides                            Sam.   expansion                                                                             oxidation                                                                           resistance                                                                          (with or                                           No.    (R.T. ˜ 1273 K)                                                                 (mg/cm.sup.2)                                                                       (mΩ · cm.sup.2)                                                      without)                                                                            remarks                                      __________________________________________________________________________    Inv.                                                                              1  12.8 × 10.sup.-6 /K                                                             3.0   7     with                                                   2  12.5 × 10.sup.-6 /K                                                             2.8   8     with                                                   3  12.7 × 10.sup.-6 /K                                                             2.8   9     with                                                   4  12.7 × 10.sup.-6 /K                                                             2.7   6     with                                                   5  12.3 × 10.sup.-6 /K                                                             1.2   4     with                                                   6  12.9 × 10.sup.-6 /K                                                             2.7   6     with                                                   7  12.9 × 10.sup.-6 /K                                                             2.1   5     with                                                   8  12.8 × 10.sup.-6 /K                                                             2.7   7     with                                                   9  12.4 × 10.sup.-6 /K                                                             2.4   5     with                                                   10 12.5 × 10.sup.-6 /K                                                             3.0   9     with                                                   11 12.6 × 10.sup.-6 /K                                                             1.5   4     with                                                   12 12.7 × 10.sup.-6 /K                                                             0.9   3     with                                                   13 12.6 × 10.sup.-6 /K                                                             1.5   4     with                                               Com.                                                                              14 12.1 × 10.sup.-6 /K                                                             1.3   13    without                                                                             *                                                15 12.9 × 10.sup.-6 /K                                                             1.9   16    without                                                                             *                                                16 12.2 × 10.sup.-6 /K                                                             2.5   20    without                                                                             *                                                17 12.7 × 10.sup.-6 /K                                                             2.3   18    without                                                                             *                                                18 12.9 × 10.sup.-6 /K                                                             2.8   19    without                                                                             *                                                19 13.3 × 10.sup.-6 /K                                                             26.4  530   without                                                20 13.5 × 10.sup.-6 /K                                                             6.4   180   without                                                21 12.8 × 10.sup.-6 /K                                                             3.0   21    without                                                                             *                                                22 12.4 × 10.sup.-6 /K                                                             0.8   10    without                                                                             *                                                23 12.4 × 10.sup.-6 /K                                                             1.1   11    without                                                                             *                                                24 12.5 × 10.sup.-6 /K                                                             1.6   17    without                                                                             *                                                25 13.8 × 10.sup.-6 /K                                                             22.6  510   without                                                                             SUS430                                           26 13.6 × 10.sup.-6 /K                                                             30.3  640   without                                                27 13.2 × 10.sup.-6 /K                                                             5.2   120   without                                                28 13.5 × 10.sup.-6 /K                                                             0.6   380   with                                               __________________________________________________________________________

Embodiment 2

Various types of Cr--W--Co--Fe system alloys according to the presentinvention were prepared as listed in Table 3. Experimental results on0.2% yield strength at 1,273K, the coefficient of thermal expansion fromroom temperature to 1,273K, weight gain due to the high temperatureoxidation and the specific resistance are listed and compared with thoseobtained from the conventional types of alloys, as seen in Table 4.

The 0.2% yield strength test at 1,273K was conducted with the hightemperature tensile testing machine, which is based on the JIS G0567(JIS: Japanese Industrial Standard) Specification "High Temperature TestMethod for Iron Steels and Heat Resisting Alloys". The tests wereconducted under the strain rate of 0.6%/min. Experimental conditions forthe high temperature weight changes due to oxidation and the specificresistance measurements were same as those used for the tests forEmbodiment 1.

From Tables 3 and 4, it was found that Cr--W--Co--Fe system alloysaccording to the present invention showed superior high temperaturemechanical properties and close values of coefficient of thermalexpansion (10˜12×10⁻⁶ /K) of stabilized zirconia. It was also evaluatedthat the present alloys exhibit superior properties of electricalconductivity and oxidation resistance to the conventional types ofalloys.

                                      TABLE 3                                     __________________________________________________________________________    Cf. Inv.:This Invention, Com.:Comparison                                      Sam.  chemical composition(wt %, bal-Fe)                                      No.   Cr W  Co                                                                              Al Y  Hf Ce La Nd  Dy B                                         __________________________________________________________________________    Inv.                                                                             31 15.1                                                                             5.2                                                                              9.9                                                                             0.11                                                                             -- -- -- -- 0.21                                                                              -- 0.002                                        32 38.2                                                                             6.1                                                                              1.2                                                                             0.13                                                                             -- -- -- -- --  0.15                                                                             --                                           33 16.9                                                                             14.2                                                                             7.3                                                                             0.26                                                                             -- -- -- 0.39                                                                             --  -- 0.006                                        34 24.1                                                                             6.3                                                                              3.4                                                                             0.64                                                                             -- -- 0.20                                                                             -- --  -- --                                           35 15.8                                                                             9.7                                                                              8.5                                                                             0.34                                                                             -- 0.12                                                                             -- -- --  -- 0.008                                        36 18.4                                                                             9.0                                                                              5.1                                                                             0.52                                                                             -- -- 0.063                                                                            0.037                                                                            --  -- 0.003                                        37 30.3                                                                             7.5                                                                              2.3                                                                             0.41                                                                             0.89                                                                             -- -- -- --  -- --                                           38 17.9                                                                             10.4                                                                             6.7                                                                             0.98                                                                             -- -- -- -- --  -- 0.010                                        39 21.5                                                                             8.0                                                                              4.6                                                                             -- -- -- -- -- --  -- --                                           40 18.1                                                                             9.3                                                                              5.8                                                                             0.18                                                                             -- -- -- -- --  -- --                                           41 19.5                                                                             7.0                                                                              2.9                                                                             -- -- -- -- -- --  -- --                                           42 18.3                                                                             9.2                                                                              5.3                                                                             -- -- -- 0.012                                                                            0.007                                                                            --  -- --                                           43 37.7                                                                             5.6                                                                              3.4                                                                             -- -- -- -- --  0.015                                                                            -- --                                           44 24.8                                                                             12.4                                                                             9.8                                                                             -- 0.55                                                                             -- -- -- --  -- --                                           45 18.2                                                                             7.5                                                                              5.4                                                                             -- -- -- -- -- --  -- 0.005                                        46 26.5                                                                             11.1                                                                             8.3                                                                             -- -- -- -- -- --  -- 0.004                                     Com.                                                                             47 17.8                                                                             9.8                                                                              --                                                                              0.54                                                                             -- -- -- -- --  -- --                                           48 18.3                                                                             7.3                                                                              --                                                                              -- -- -- -- -- --  -- --                                           49 15.6                                                                             14.8                                                                             --                                                                              -- -- -- -- -- --  -- --                                           50 17.0                                                                             22.0                                                                             --                                                                              -- -- -- -- -- --  -- --                                           51 25.4                                                                             7.8                                                                              --                                                                              -- -- -- 0.020                                                                            0.008                                                                            --  -- --                                           52 17.8                                                                             9.1                                                                              --                                                                              2.01                                                                             -- -- -- -- --  -- --                                           53 17.1                                                                             -- --                                                                              -- -- -- -- -- --  -- --                                           54 24.3                                                                             5.8                                                                              --                                                                              -- -- -- -- -- 0.31                                                                              0.12                                                                             0.009                                        55 20.6                                                                             8.9                                                                              --                                                                              -- -- 0.08                                                                             -- -- --  -- 0.005                                        56 39.5                                                                             5.3                                                                              --                                                                              -- 0.68                                                                             -- -- -- --  -- --                                           57 32.1                                                                             7.4                                                                              --                                                                              -- -- -- -- -- --  -- 0.007                                     __________________________________________________________________________

                  TABLE 4                                                         ______________________________________                                        Cf. Inv.:This Invention, Com.:Comparison                                                                   weight gain                                                                           the                                                                   due to the                                                                            increments                                                coefficient of                                                                            high    in the                                           0.2% yield                                                                             thermal     temperature                                                                           specific                                 Sam.    strength expansion   oxidation                                                                             resistance                               No.     (kgf/mm.sup.2)                                                                         (R.T. ˜ 1273 K)                                                                     (mg/cm.sup.2)                                                                         (mΩ · cm.sup.2)           ______________________________________                                        Inv. 31     3.1      12.9 × 10.sup.-6 /K                                                               1.3     9                                           32     2.7      12.3 × 10.sup.-6 /K                                                               2.6     8                                           33     2.7      12.8 × 10.sup.-6 /K                                                               1.9     7                                           34     2.9      12.5 × 10.sup.-6 /K                                                               2.8     4                                           35     2.9      12.5 × 10.sup.-6 /K                                                               2.3     5                                           36     2.8      12.2 × 10.sup.-6 /K                                                               0.8     3                                           37     2.8      12.7 × 10.sup.-6 /K                                                               1.5     4                                           38     2.7      12.6 × 10.sup.-6 /K                                                               3.0     9                                           39     3.2      12.3 × 10.sup.-6 /K                                                               2.3     18                                          40     3.0      12.4 × 10.sup.-6 /K                                                               1.7     9                                           41     2.8      12.6 × 10.sup.-6 /K                                                               2.6     21                                          42     2.8      12.5 × 10.sup.-6 /K                                                               1.6     13                                          43     2.6      12.7 × 10.sup.-6 /K                                                               2.1     19                                          44     3.1      12.8 × 10.sup.-6 /K                                                               2.2     17                                          45     3.0      12.5 × 10.sup.-6 /K                                                               3.0     19                                          46     3.1      12.7 × 10.sup.-6 /K                                                               2.8     20                                     Com. 47     1.7      12.5 × 10.sup.-6 /K                                                               3.3     25                                          48     1.2      12.6 × 10.sup.-6 /K                                                               4.9     19                                          49     1.3      12.9 × 10.sup.-6 /K                                                               5.2     32                                          50     1.5      13.5 × 10.sup.-6 /K                                                               11.1    78                                          51     1.9      12.5 × 10.sup.-6 /K                                                               1.8     16                                          52     1.1      13.2 × 10.sup.-6 /K                                                               1.0     400                                         53     1.0      13.8 × 10.sup.-6 /K                                                               22.7    530                                         54     1.2      12.8 × 10.sup.-6 /K                                                               2.5     18                                          55     1.4      12.4 × 10.sup.-6 /K                                                               1.3     15                                          56     1.6      12.9 × 10.sup.-6 /K                                                               2.8     23                                          57     1.3      12.8 × 10.sup.-6 /K                                                               10.7    20                                     ______________________________________                                    

Embodiment 3

Various Cr--W--(Ti,Zr,Hf)--Fe system alloys according to the presentinvention were fabricated with respective chemical compositions, aslisted in Table 5. The 0.2% yield strength at 1,273K, the coefficient ofthermal expansion from room temperature to 1,273K, the weight gain andspecific resistance at the high temperature were measured and comparedto results obtained from the conventional types of alloys, as seen inTable 6.

From Tables 5 and 6, it is clearly found that Cr--W--(Ti,Zr,Hf)--Fesystem alloys of the present invention demonstrated superior hightemperature mechanical properties and specific resistance to theconventional type of alloys. They also exhibit close values ofcoefficient of thermal expansion (10˜12×10⁻⁶ /K) of stabilized zirconia.Moreover, they also show a superiority of electrical conductivity andresistance against the high temperature oxidation to the conventionaltypes of alloys.

                                      TABLE 5                                     __________________________________________________________________________    Cf. Inv.:This Invention, Com.:Comparison                                      Sam.  chemical composition(wt %, bal-Fe)                                      No.   Cr W  Co Ti Zr Hf Al Y  Ce  La Nd  Dy B                                 __________________________________________________________________________    Inv.                                                                             61 17.7                                                                             13.6                                                                             -- 0.17                                                                             0.35                                                                             -- -- -- --  -- --  -- --                                   62 20.8                                                                             5.4                                                                              -- 1.8                                                                              -- 0.21                                                                             0.32                                                                             -- --  -- --  -- --                                   63 25.3                                                                             11.8                                                                             9.5                                                                              -- 0.13                                                                             -- -- -- --  -- 0.11                                                                              -- --                                   64 17.8                                                                             9.0                                                                              5.8                                                                              4.9                                                                              -- -- -- -- --  -- --  0.08                                                                             --                                   65 18.0                                                                             8.9                                                                              6.1                                                                              -- 0.68                                                                             -- 0.51                                                                             -- --  -- --  -- --                                   66 20.4                                                                             6.2                                                                              -- 0.42                                                                             2.7                                                                              0.36                                                                             -- -- --  -- --  -- 0.002                                67 32.1                                                                             7.1                                                                              -- 1.1                                                                              1.4                                                                              2.2                                                                              0.28                                                                             -- --  -- --  -- 0.007                                68 19.5                                                                             14.3                                                                             5.0                                                                              -- -- 2.8                                                                              -- -- --  -- --  -- 0.008                                69 18.8                                                                             9.1                                                                              5.8                                                                              -- 1.0                                                                              -- 0.97                                                                             -- --  -- --  -- 0.004                                70 23.6                                                                             6.7                                                                              2.9                                                                              -- 4.6                                                                              -- 0.66                                                                             0.92                                                                             --  -- --  -- --                                   71 39.8                                                                             7.4                                                                              1.1                                                                              1.3                                                                              -- 0.21                                                                             0.14                                                                             -- --  -- --  -- --                                   72 16.2                                                                             5.1                                                                              9.2                                                                              -- 3.4                                                                              -- 0.17                                                                             --  0.072                                                                            0.033                                                                            --  -- 0.003                                73 15.7                                                                             14.7                                                                             7.4                                                                              -- -- 1.7                                                                              0.69                                                                             -- 0.28                                                                              -- --  -- 0.005                                74 16.0                                                                             9.5                                                                              8.0                                                                              -- -- 4.3                                                                              0.36                                                                             -- --  -- --  -- 0.010                                75 29.9                                                                             7.6                                                                              2.5                                                                              0.32                                                                             1.8                                                                              -- 0.45                                                                             -- --  0.31                                                                             --  -- --                                   76 22.2                                                                             8.2                                                                              4.5                                                                              0.80                                                                             1.1                                                                              1.2                                                                              0.11                                                                             -- --  -- --  -- 0.009                                77 20.1                                                                             6.9                                                                              3.2                                                                              2.5                                                                              -- -- -- -- --  -- --  -- --                                   78 32.4                                                                             5.8                                                                              4.1                                                                              0.60                                                                             2.1                                                                              1.2                                                                              -- -- --  -- --  -- 0.006                                79 15.2                                                                             10.3                                                                             -- 1.2                                                                              0.52                                                                             -- -- 0.24                                                                             --  -- 0.01                                                                              -- --                                   80 34.5                                                                             6.4                                                                              -- -- 0.29                                                                             0.84                                                                             0.47                                                                             -- 0.06                                                                              -- --  0.03                                                                             --                                   81 18.4                                                                             7.8                                                                              -- 2.1                                                                              -- 2.4                                                                              -- -- --  0.05                                                                             0.08                                                                              -- 0.002                                82 27.5                                                                             5.2                                                                              -- 0.04                                                                             0.08                                                                             0.16                                                                             0.56                                                                             0.31                                                                             0.10                                                                              0.14                                                                             --  -- 0.008                             Com.                                                                             83 15.8                                                                             9.7                                                                              8.5                                                                              -- -- 0.12                                                                             0.34                                                                             -- --  -- --  -- 0.008                                84 18.4                                                                             9.0                                                                              5.1                                                                              -- -- -- 0.52                                                                             -- 0.63                                                                              0.037                                                                            --  -- 0.003                                85 30.3                                                                             7.5                                                                              2.3                                                                              -- -- -- 0.41                                                                             0.89                                                                             --  -- --  -- --                                   86 37.7                                                                             5.6                                                                              3.4                                                                              -- -- -- -- -- --  --  0.015                                                                            -- --                                   87 17.8                                                                             9.8                                                                              -- -- -- -- 0.54                                                                             -- --  -- --  -- --                                   88 17.8                                                                             9.1                                                                              -- -- -- -- 2.01                                                                             -- --  -- --  -- --                                   89 17.1                                                                             -- -- -- -- -- -- -- --  -- --  -- --                                   90 35.5                                                                             7.3                                                                              -- -- -- -- -- -- --  -- --  -- --                                   91 24.3                                                                             11.5                                                                             -- -- -- -- -- -- --  -- --  -- 0.007                                92 16.2                                                                             5.5                                                                              -- -- -- -- -- -- --  -- 0.22                                                                              0.13                                                                             --                                   93 19.3                                                                             14.8                                                                             -- -- -- -- -- -- --  -- --  0.56                                                                             0.004                             __________________________________________________________________________

                                      TABLE 6                                     __________________________________________________________________________    Cf. Inv.:This Invention, Com.:Comparison                                                          weight gain                                                                         the                                                             coefficient of                                                                        due to the                                                                          increment                                                 0.2% yield                                                                          thermal high  in the                                                    strength                                                                            expansion                                                                             temperature                                                                         specific                                                                            specific                                      Sam.  (kgf/mm.sup.2)                                                                      (1/K)   oxidation                                                                           resistance                                                                          resistance                                    No.   (at 1273 K)                                                                         (R.T. ˜ 1273 K)                                                                 (mg/cm.sup.2)                                                                       (mΩ · cm.sup.2)                                                      (mΩ · cm.sup.2)                __________________________________________________________________________    Inv.                                                                             61 1.5   12.3 × 10.sup.-6 /K                                                             1.5   6     21                                               62 1.2   12.4 × 10.sup.-6 /K                                                             0.8   7     18                                               63 3.2   12.8 × 10.sup.-6 /K                                                             2.3   9     25                                               64 2.7   12.5 × 10.sup.-6 /K                                                             0.8   5     17                                               65 3.1   12.4 × 10.sup.-6 /K                                                             2.2   8     22                                               66 1.4   12.5 × 10.sup.-6 /K                                                             1.8   7     15                                               67 1.3   12.8 × 10.sup.-6 /K                                                             1.0   8     14                                               68 2.6   12.6 × 10.sup.-6 /K                                                             1.9   6     17                                               69 2.9   12.2 × 10.sup.-6 /K                                                             2.1   7     18                                               70 2.8   12.5 × 10.sup.-6 /K                                                             1.5   4     21                                               71 2.8   12.3 × 10.sup.-6 /K                                                             1.1   3     17                                               72 3.0   12.9 × 10.sup.-6 /K                                                             0.7   2     16                                               73 2.8   12.8 × 10.sup.-6 /K                                                             1.2   3     15                                               74 2.8   12.5 × 10.sup.-6 /K                                                             0.7   4     20                                               75 2.9   12.7 × 10.sup.-6 /K                                                             0.8   4     15                                               76 3.1   12.3 × 10.sup.-6 /K                                                             1.7   9     17                                               77 2.9   12.6 × 10.sup.-6 /K                                                             1.3   9     16                                               78 2.5   12.7 × 10.sup.-6 /K                                                             0.9   5     15                                               79 1.9   12.4 × 10.sup.-6 /K                                                             0.9   4     14                                               80 1.6   12.6 × 10.sup.-6 /K                                                             0.7   2     11                                               81 1.8   12.5 × 10.sup.-6 /K                                                             0.8   3     13                                               82 1.6   12.5 × 10.sup.-6 /K                                                             0.7   4     12                                            Com.                                                                             83 2.9   12.5 × 10.sup.-6 /K                                                             2.3   5     46                                               84 2.8   12.2 × 10.sup.-6 /K                                                             0.8   3     42                                               85 2.8   12.7 × 10.sup.-6 /K                                                             1.5   4     57                                               86 2.6   12.7 × 10.sup.-6 /K                                                             2.1   19    62                                               87 1.7   12.5 × 10.sup.-6 /K                                                             3.3   25    58                                               88 1.1   13.2 × 10.sup.-6 /K                                                             1.0   400   70                                               89 1.0   13.8 × 10.sup.-6 /K                                                             22.7  530   65                                               90 1.5   12.9 × 10.sup.-6 /K                                                             18.9  280   61                                               91 1.8   12.8 × 10.sup.-6 /K                                                             20.6  310   63                                               92 1.3   12.5 × 10.sup.-6 /K                                                             2.9   23    52                                               93 1.4   12.6 × 10.sup.-6 /K                                                             2.4   21    54                                            __________________________________________________________________________

INDUSTRIAL APPLICABILITY

Since it was found that various types of Cr--W--Fe system alloysaccording to the present invention exhibit close values of coefficientof thermal expansion (10˜12×10⁻⁶ /K) of stabilized zirconia, excellenthigh temperature mechanical properties, superior electrical conductivityand oxidation resistance to the conventional types of alloys, thepresent alloys can be used as an intermediate layer between thesubstrate material and the thermal barrier coating of stabilizedzirconia system ceramic, a separator of solid oxide fuel cell in whichstabilized zirconia is employed as an electrolyte, or the like.Furthermore, the present alloys can be co-used with any materials whosecoefficient of thermal expansion are close to those of stabilizedzirconia.

Since the oxidation resisting low thermal expansion alloys of thepresent invention such as Cr--W--Al--Fe system, Cr--W--Al--B--Fe system,Cr--W--Al--M--Fe system and Cr--W--Al--B--M--Fe system alloys can beprepared by an appropriate controlling of adding amounts of Cr and Alelements, a surface oxide layer comprising of Cr₂ O₃ containing Al₂ O₃and internally dispersed Al₂ O₃ can be formed. Because of these surfaceoxide and internal oxide formation, the present alloys exhibit closervalues of coefficient of thermal expansion to those of stabilizedzirconia than conventional stainless steels, resulting in providing abetter thermal matching. It is also found that the present alloys showexcellent electrical conductivity. Hence, these newly developed alloysare good oxidation resisting materials which can be used in variousapplications.

The specific resistance (which can be obtained by the increments inresistance after treatments in the air at 1,273K for 490 hoursmultiplied with the total surface area) of the various alloys from theCr--W--Al--Fe system according to the present invention show an equal toor less value than those obtained from conventional types of alloys.These alloys exhibit excellent high temperature electrical conductivity.

Alloys from Cr--W--Co--Fe system, Cr--W--Co--Al--Fe system,Cr--W--Co--B--Fe system, and Cr--W--Co--Al--B--Fe system show closevalues of coefficient of thermal expansion of stabilized zirconia andmore than 2.0 kgf/mm² of 0.2% yield strength at 1,273K, indicating thatthe present alloys exhibit excellent high temperature mechanicalproperties, the oxidation resistance, and the electrical conductivity.

Alloys from Cr--W--(Ti,Zr,Hf)--Fe system, Cr--W--(Ti,Zr,Hf)--B--Fesystem, Cr--W--(Ti,Zr,Hf)--M--Fe system, Cr--W--(Ti,Zr,Hf)--B--M--Fesystem, Cr--W--(Ti,Zr,Hf)--Al--Fe system, Cr--W--(Ti--Zr,Hf)--Al--B--Fesystem, Cr--W--(Ti,Zr,Hf)--Al--M--Fe system,Cr--W--(Ti,Zr,Hf)--Al--B--M--Fe system, Cr--W--(Ti--Zr,Hf)--Co--Fesystem, Cr--W--(Ti,Zr,Hf)--Co--B--Fe system,Cr--W--(Ti,Zr,Hf)--Co--M--Fe system, Cr--W--(Ti,Zr,Hf)--Co--B--M--Fesystem, Cr--W--(Ti,Zr,Hf)--Al--Co--Fe system,Cr--W--(Ti,Zr,Hf)--Al--Co--B--Fe system,Cr--W--(Ti--Zr,Hf)--Al--Co--M--Fe system andCr--W--(Ti,Zr,Hf)--Al--Co--B--M--Fe system according to the presentinvention show close values of coefficient of thermal expansion ofstabilized zirconia and exhibit excellent high temperature mechanicalproperties, the oxidation resistance, and the electrical conductivity.

As a result, these alloy systems of the present invention can promise aprolonging the lives of various types of plant machinery being operatedat elevated temperatures including gas turbine and a scaling-up of solidoxide fuel cell. Furthermore, since these materials are metallicmaterials, the present materials (metallic alloys) are much easilymachined than oxides, so that the productivity and low cost-performanceof solid oxide fuel cell can be realized.

While this invention has been described in detail with respect topreferred embodiment and examples, it should be understood that theinvention is not limited to that precise embodiments; rather manymodifications, and variations would present themselves to those of skillin the art without departing from the scope and spirit of thisinvention, as defined in the appended claims.

What is claimed is:
 1. A low thermal expansion alloy comprisingCr--W--Fe based alloy selected from the groups consistingofCr--W--Al--Fe, Cr--W--Al--B--Fe, Cr--W--Al--M--Fe,Cr--W--Al--B--M--Fe, Cr--W--Co--Fe, Cr--W--Co--B--Fe, Cr--W--Co--M--Fe,Cr--W--Co--B--M--Fe, Cr--W--Co--Al--Fe, Cr--W--Co--Al--B--Fe,Cr--W--Co--Al--M--Fe, Cr--W--Co--Al--B--M--Fe, Cr--W--(Ti, Zr, Hf)--Fe,Cr--W--(Ti, Zr, Hf)--B--Fe, Cr--W--(Ti, Zr, Hf)--M--Fe, Cr--W--(Ti, Zr,Hf)--B--M--Fe, Cr--W--(Ti, Zr, Hf)--Al--Fe, Cr--W--(Ti, Zr,Hf)--Al--B--Fe, Cr--W--(Ti, Zr, Hf)--Al--M--Fe, Cr--W--(Ti, Zr,Hf)--Al--B--M--Fe, Cr--W--(Ti, Zr, Hf)--Co--Fe, Cr--W--(Ti, Zr,Hf)--Co--B--Fe, Cr--W--(Ti, Zr, Hf)--Co--M--Fe, Cr--W--(Ti, Zr,Hf)--Co--B--M--Fe, Cr--W--(Ti, Zr, Hf)--Al--Co--Fe, Cr--W--(Ti, Zr,Hf)--Al--Co--B--Fe, Cr--W--(Ti, Zr, Hf)--Al--Co--M--Fe and Cr--W--(Ti,Zr, Hf)--Al--Co--B--M--Fe,the low thermal expansion alloy havinglimitations for each alloying element comprising 15-40 wt % of Cr, 5-15wt % of W, 0.1-1 wt % of Al, 1-10 wt % of Co, 0.1-5 wt % of at least oneelement selected from Ti, Zr and Hf, 0.001-0.01 wt % of B, 0.01-1 wt %of M which is at least one element selected from Y, Hf, Ce, La, Nd andDy and the balance being Fe and unavoidable impurities, wherein when Hfamong Ti, Zr and Hf is solely used, the alloy can contain 1 to 5 wt % ofHf and only when at least one element out of Ti and Zr is used, Hf canbe selected as M.
 2. A low thermal expansion alloy according to claim 1,wherein an average thermal expansion coefficient at a temperature offrom room temperature to 1000° C. is more than 12×10⁶ /K but less than13×10⁶ /K.
 3. A low thermal expansion alloy according to claim 1,wherein when containing Co, 0.25 yield strength of the alloy at atemperature of 1000° C. is more than 2.0 kgf/mm².
 4. A low thermalexpansion alloy according to claim 1, wherein when containing Al, thealloy is provided on its surface with an oxide layer composed of Cr₂ O₃containing Al₂ O₃ and in the inside with a dispersion layer of Al₂ O₃.5. A low thermal expansion alloy according to claim 1, wherein aspecific resistance of the alloy is less than 10 m Ω/cm², the specificresistance of which is expressed by a value resulting frommultiplication of an increment of the resistance obtained after aheat-treatment at 1000° C. for 1000 hours in atmosphere by the surfacearea of an alloy species.